The invention relates to well logging for evaluating the resistivity of earth formations through which a borehole passes, and more particularly to an electrical logging method and apparatus using electrodes.
A well logging device with electrodes that has been commercially available for many years and that is known under the name "Dual Laterolog" is described in U.S. Pat. No. 3,772,589 (Scholberg). That device comprises an array of annular electrodes used for sending electrical measurement currents into the formations for the purpose of measuring their resistivity. The measurement currents are focused in an annular zone having the form of a disk perpendicular to the borehole axis by means of auxiliary currents emitted by guard electrodes. That device includes measuring deep resistivity of the earth formations (LLd mode) and measuring shallower resistivity of the earth formations (LLs mode) by emitting currents at different frequencies, typically 35 Hz and 280 Hz.
A drawback of that device is that its longitudinal resolution (i.e. in the longitudinal direction of the borehole) is poor, being about one meter. In addition, in some cases and in particular in boreholes that are deviated or horizontal, the annular zone scanned by the device around the borehole comprises different layers of earth formation and gives an average measurement that has little meaning. It is therefore desirable to obtain resistivity measurements in a plurality of azimuthal directions around the borehole.
Logging apparatuses are known that detect the resistivity of formations in a plurality of azimuthal directions around the axis of the borehole. Those devices seek to obtain information about fractures or about the dip of formations. For example, British patent 928 583 (British Petroleum Company Limited) describes an array of azimuthal measurement electrodes distributed circumferentially around the periphery of a logging sonde. A guard electrode which surrounds the measurement electrodes enables an auxiliary current to be emitted for focusing the currents emitted by each of the measurement electrodes.
In such a sonde, measurement current focusing is passive, and this focusing is obtained by emitting the various currents via electrodes that are short-circuited together. Unfortunately, focusing must be particularly effective since the measurement electrodes are disposed on the body of the sonde and thus may be spaced-apart long distances, up to about 10 cm, from the wall of the borehole, such spacing distances being possibly different in different directions when the sonde is not centered in the borehole. A defect of this passive focusing technique is the often inadequate quality of its focusing.
French patent FR 2 611 920 (CNRS) describes a logging sonde in which correction means are proposed acting on the potential of the current electrodes in order to improve focusing. The sonde includes monitor electrodes disposed at a certain distance ahead of the current electrodes and circuits that are responsive to the potentials detected by said monitor electrodes to control the measurement currents. It is difficult and complicated to make such a sonde, which requires concentric rings of electrodes.